1. Field of the Invention
The present invention generally relates to an optical pulse delivery system for various types of optical devices, such as an optical measurement system, requiring ultrashort pulses with high peak power. More particularly, the present invention relates to an optical pulse delivery system which employs an optical fiber and which is capable of compensating for various dispersion effects within the system (including those within the optical device, if desired) in order to deliver high peak power pulses.
2. Description of the Related Art
Ultrashort optical pulse sources are presently known to be capable of creating pulses having pulse widths of picosecond to sub-picosecond duration at a variety of wavelengths, pulse energies, and repetition rates up to the GHz regime. Such optical pulse sources are commonly used in measurement and imaging applications that require time gating or excitation by a high peak power or high intensity. Ultrashort optical pulses provide both high spatial and high temporal resolutions, as well as high peak powers in a focusable beam necessary for the excitation of certain non-linear events (such as the excitation of a multi-photon fluorescent medium). These capabilities find use in applications including biological and medical imaging, metrology, terahertz generation, photoconductive and electro-optical sampling, and optical time domain reflectometers.
Current techniques for the delivery of ultrashort optical pulses to a device under test or a measurement point include the use of optical components such as mirrors, lenses, optical fiber, beamsplitters, and dichroic elements. Ultrashort optical pulses passing through a delivery system made of such elements will experience a change in peak power as well as distortions in their temporal profile. These distortions may result in a reduction in resolution, or a degradation in signal-to-noise ratio. The changes in peak power and temporal shape of an ultrashort optical pulse signal propagating through an optical system are caused by losses and dispersion. In addition, at high peak powers, non-linear effects can distort the optical pulse.
An ultrashort optical pulse is made up of a certain range of optical frequencies (or wavelengths), which constitutes its bandwidth. The shortest pulse for a given bandwidth (the bandwidth-limited pulse) has all of its frequency components perfectly overlapped in time. In propagation through a system, the different wavelength components of a pulse experience different delays. These different delays will cause the above-mentioned distortion in temporal shape and change in peak power of ultrashort optical pulses. The result is a frequency chirped pulse, where instantaneous frequency is a function of time along the pulse.
Propagation through a common optically transparent material used to deliver optical signals, is such as glass, will generally result in very small loss. However, due to the frequency dependent refractive index n(.nu.) of the medium, which gives the velocity, v, of propagation of the optical signal by the relationship v=c/ n(.nu.), where c is the speed of light in a vacuum, different wavelengths, .lambda., experience different velocities in the material, where wavelength is related to frequency by .lambda.=/c.nu.. This effect is referred to as chromatic dispersion. Through the interaction of a pulsed optical signal and such a material, pulse broadening can occur due to group velocity dispersion (GVD). This effect causes the lower frequency components and the higher frequency components of the bandwidth to arrive at different times after passing through the dispersing medium. The effect may be that the lower frequency components arrive earlier or later, depending upon the sign of the dispersion. In glass, for wavelengths shorter than the zero-dispersion wavelength (.about.1300 nm), the sign of the dispersion is positive, and higher frequencies of the optical pulse travel more slowly than lower frequencies. Above the zero-dispersion wavelength, the sign of the dispersion is negative, and lower frequencies of the optical pulse travel more slowly than higher frequencies. Therefore, any optical element through which the ultrashort optical pulse is transmitted may potentially have a distorting effect.
Dispersion manipulation may be performed with several well known optical elements and systems. These include glass prisms, diffraction gratings, fiber gratings, and optical fiber. These elements allow for both signs of dispersion to be reached at any wavelength, as well as allowing for compensation of frequency chirp. Glass prism pairs can be used to create a dispersive delay line, where, by varying the distance between the two prisms, different amounts of dispersion can be achieved. Similarly, using either reflective or transmissive diffraction gratings, variable amounts of both positive and negative dispersion can be provided. Fiber gratings are chirped Bragg gratings written in the core of an optical fiber. In a chirped fiber grating, dispersion is achieved by reflecting different wavelengths at different locations in space, thereby adding different time shifts to different wavelength components. Specialty fibers can be made for wavelengths longer than .about.1300 nm. These fibers use waveguide dispersion in conjunction with material dispersion to create tailored dispersion which may be positive, negative, or close to zero.
Of the commonly used optics for beam steering in an optical system, optical fibers are a convenient method of delivery in practical systems, particularly those where the laser source is bulky. Optical fibers offer increased reliability and robustness, by allowing for stable pre-alignment of components. By providing confinement of the laser light, optical fiber delivery allows for placement of the laser source in more diverse environments than the typical laser laboratory, as well as allowing for convenient placement of the source of light with respect to the rest of the system, providing more flexibility in system design. Additionally, the optical fiber can be disconnected without disturbing the alignment of the laser source and the optical device; thus, the two systems can be pre-aligned and shipped separately in different boxes. However, optical fibers can distort the temporal profile of ultrashort optical pulses, as described below.
Optical fibers can be characterized as being single-mode (capable of propagating a single spatial mode) or multi-mode (capable of supporting the propagation of many spatial modes) for wavelength .lambda.. Considering the single-mode case, the properties of optical fiber pulse propagation include: a frequency dependent loss, material dispersion giving rise to pulse broadening, and waveguide dispersion. At the "zero dispersion" point where the material dispersion changes sign (for instance, in standard telecommunications fiber, at .about.1300 nm) pulses may propagate without significant broadening. However, as the material dispersion effect decreases, waveguide dispersion becomes significant, arising from the confinement of the mode at the core-cladding interface. In multi-mode fiber, the situation is further complicated by the addition of many spatial modes which may produce further temporal broadening. However, multi-mode fiber is of interest in a number of applications due to its higher tolerance to misalignment.
In long-haul fiber-optic telecommunications systems, there exists the problem of high bit-error-rates due to broadening of optical signal pulses along the long optical fiber delivery lengths. This problem has been addressed using various schemes, including dispersion compensation by using specially designed optical fibers, pre-chirping of the pulses, possibly using optical fiber gratings for either of these techniques. However, the peak powers of the signals used in these systems are below the onset of non-linear effects; these systems do not address the delivery of high peak power (high peak power is herein defined as &gt;1 kW) pulses through the optical fiber.
One system that requires the delivery of optimized pulse widths is that of a two-photon laser microscope. As disclosed by Denk, et al. in U.S. Pat. No. 5,034,613, such a system comprises a laser scanning microscope, a fluorophore having the appropriate emission with long wavelength (red or infrared) illumination as a stain for a sample, a picosecond or sub-picosecond laser source of appropriate wavelength, a detector for the emission of the fluorophore, and signal processing provided by a computer. Although several different sources have been used to provide ultrashort pulses including Ti:Sapphire and Cr:LiSAF, the delivery of the high peak power pulses has been made in "free space". In one such system reported by M. Muller et al. in "Measurement of Femtosecond Pulses in the Focal Point of a High-Numerical-Aperture Lens by Two-Photon Absorption", Optics Letters, Vol. 20, No. 9 (1995), the microscope objective was found to distort the pulses incident upon the lens, broadening the pulses appreciably.
Another example of an application that requires optimized pulses is optical metrology. In optical metrology, measurement of a physical parameter is made in a non-destructive, non-contact manner using an optical measuring device. The device includes a source of optical pulses, a delivery mechanism, and a probe that is inserted in such a way that the light source illuminates an object to be measured and its reflection is captured for signal processing. A convenient manner of delivering the light from the pulsed source is to provide the light through optical fiber which is incorporated into the probe. The probe may then be constructed to be convenient for mounting into a measurement system, bringing the probe into approximate contact with the sample. In this system, the resolution is dependent upon achieving the high peak power (shortest) pulse at the measurement point, which is in the doubling crystal of an auto-correlator or cross-correlator within the measurement system. However, optical fiber delivery alone creates a condition that is not optimized to deliver the shortest pulses to that point.
Other systems have utilized some form of optical fiber delivery of modelocked pulses to a sample under investigation. In particular, in International Application No. PCT/US92/03536, Huang et al. describe a system for optical coherence domain reflectometry, which includes optical fiber delivery of a short coherence length source to the sample under measurement. Such a source may be either a broad-bandwidth, superluminescent source or an ultrashort pulse (and thus broad-bandwidth) modelocked source. In the measurement technique, there exists a reference and a sample path which are arranged as an interferometer (the two paths must create optical interference in order to obtain the measurement information). A necessary condition for this interference to occur is that the optical path lengths from the source to the reference and from the source to the sample must be nearly equal, a condition described by: L.sub.ref -L.sub.sample.about.L.sub.coherence, where L.sub.ref is the optical path length from the source to the reference, L.sub.sample, is the optical path length from the source to the sample, and L.sub.coherence is the coherence length of the optical source. This condition must be true for all wavelengths. Here, it is recognized that, if the length of optical fiber in one leg of the system is shorter than the other, for say light on the blue side of the spectrum, another optical material of known higher GVD and shorter length may be added to the shorter arm to equalize these optical path lengths, thus compensating the relative length as a function of wavelength. This compensation is performed to ensure that each component wavelength in the broad bandwidth light arrives at the end points of the two optical paths at the same time (i.e. the shortest wavelengths, the middle wavelengths, and the longest wavelengths arrive at the same time). For use with a modelocked source, the pulse widths need not be short. However, the chirp is the same for pulses from each path.
In a similar system reported by Bouma et al. in "High-Resolution Optical Coherence Tomographic Imaging Using a Mode-Locked Ti:Al.sub.2 O.sub.3 Laser Source", Optics Letters, Vol. 20, No. 13 (1995), optical coherence tomographic imaging is performed using a modelocked Cr:Forsterite laser. Again, the optical bandwidth of the laser is of primary importance to the imaging technique.
Therefore, the limited bandwidth of the Cr:Forsterite was enhanced by using the well-known method of generating bandwidth in optical fiber using self-phase modulation. Thus, an optical fiber was added between the output of the laser and the input of the imaging system, providing the side benefit of ease of alignment. Here, however, there was no need, nor intent, to provide optimized pulse widths to the sample under investigation. In fact, to achieve the shortest pulses it is normally desired to avoid self-phase modulation.
Another system that takes advantage of optical fiber delivery is described by Harris in U.S. Pat. No. 5,120,953. Here, light is delivered to a sample in a scanning confocal microscope via an optical fiber, and the back-scattered signal generated at the sample is collected in the same optical fiber for detection. The optical fiber is used to eliminate rigid positional requirements on the confocal imaging optics in the path of the microscope and to act as a spatial filter for the input light mode and the back-scattered signal light. There is no concern for the pulse width of the light in such a system, due to the fact that single-photon fluorescence is the source of the signal, which fluorescence is proportional to the average power rather than intensity of the light incident upon the sample. That is why CW lasers rather than pulsed sources are used for this application.
In an optical measurement system such as the scanning confocal microscope, optical fiber delivery is particularly advantageous. A particular measurement system might incorporate an ultrafast light source coupled to an optical system via a delivery fiber. In the case of two-photon microscopy, it is essential to deliver to the sample light pulses having high peak power with low total energy. The laser intensity (i.e. W/cm.sup.2) must be high enough for two photon absorption to proceed at an acceptable rate. However, above a certain energy level, pulses can cause photobleaching and possibly damage the sample. Accordingly, there is a need to provide a measurement system with an ultrasbort pulse light source which delivers a short duration high peak power but low total energy pulse.